Easy-adhesive for solar cell rear surface protection sheet, solar cell rear surface protection sheet, and solar cell module

ABSTRACT

To overcome a problem in related art and to provide an easy-adhesive having an excellent adhesive property and an excellent adhesive durability for a solar cell rear surface protection sheet, a solar cell rear surface protection sheet, and a solar cell module formed by using the solar cell rear surface protection sheet. An easy-adhesive for a solar cell rear surface protection sheet according to the present invention exhibits a specific glass transition temperature, a specific number average molecular weight, and a specific hydroxyl value. Further, the easy-adhesive for a solar cell rear surface protection sheet contains a (meth)acrylic-based copolymer (A) that contains a carbon-carbon double bond in a side chain at a specific rate, and a polyisocyanate compound (B) at a specific rate.

This application is a U.S. National Stage Application of PCIInternational Patent Application No. PCT/JP2011/003792, which was filedon Jul. 4, 2011 and claims priority to Japanese Patent Application No.2010-154392, which was filed Jul. 7, 2010, the disclosure of each ofwhich is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to an easy-adhesive for a solar cell rearsurface protection sheet, and in particular to an adhesive having anexcellent adhesive property and an excellent easy-adhesive durabilityfor a solar cell rear surface protection sheet. Further, the presentinvention relates to a solar cell rear surface protection sheet formedby using the easy-adhesive for a solar cell rear surface protectionsheet. Further, the present invention also relates to a solar cellmodule formed by using the sheet.

BACKGROUND ART

In recent years, because of the increased awareness of environmentalissues, solar cells have attracted attention as clean energy sourcesthat cause no environmental pollution. Therefore, solar cells have beendiligently studied for the use of solar energy as useful energyresources, and thereby have been commercialized.

There are various forms of solar cell elements. Typical examples of suchsolar cell elements include a crystalline silicon solar cell element, apolycrystalline silicon solar cell element, an amorphous silicon solarcell element, a copper indium selenide solar cell element, and acompound semiconductor solar cell element. Among them, thepolycrystalline silicon solar cell element, the amorphous silicon solarcell element, and the compound semiconductor solar cell element arerelatively inexpensive and can be manufactured in a large size.Therefore, they have been actively studied and developed in variousfields. Further, among these solar cell elements, a thin-film solar cellelement, which is typified by an amorphous silicon solar cell elementthat is obtained by laminating silicon on a conductive metal substrateand forming a transparent conducive layer on the laminate, islightweight and excellent in the impact resistance and the flexibility.Therefore, the thin-film solar cell element has been regarded as apromising solar cell element as the form of solar cell in the future.

Among the solar cell modules, a simple one has such a structure that asealing agent and a glass plate are successively laminated on both sidesof a solar cell element. Since the glass plate is excellent in thetransparency, the weatherproof property, and the friction resistance, itis commonly used as protection material on the solar-light-receivingside at the present time. However, for the non-light-receiving side thatdoes not need to be transparent, various solar cell rear surfaceprotection sheets (hereinafter also referred to as “rear surfaceprotection sheets”) other than the glass plate have been proposed inconsideration of the cost, the safety, and the workability (for example,Patent literature 1). Therefore, the glass plates are beginning to bereplaced by rear surface protection sheets. Further, ethylene-vinylacetate copolymer (hereinafter referred to as “EVA”) having hightransparency and excellent moisture resistance is usually used for thesealing agent.

Examples of the rear surface protection sheet include (i) a single-layerfilm such as a polyester film, (ii) a film obtained by forming avapor-deposition layer of a metal oxide or a nonmetal oxide on apolyester film or the like, and (iii) a multilayer film obtained bylaminating films such as a polyester film, a fluorine-based film, anolefin film, and an aluminum foil.

Various properties can be added to a rear surface protection sheethaving a multilayer structure because of its multilayer structure. Forexample, an insulating property can be added by using a polyester film.Further, a water vapor barrier property can be added by using analuminum foil (see Patent literatures 2 to 4). What kind of the rearsurface protection sheet is actually used may be determined asappropriate depending on the product/use in which the solar cell moduleis used.

Among those various properties required for the rear surface protectionsheet, an adhesive property with a sealing agent and an adhesivedurability are fundamental and important required properties. If theadhesive property with the sealing agent is unsatisfactory, the rearsurface protection sheet could be pealed and the solar cell cannot beprotected from moisture and other external factors, and thus leading tothe deterioration in the output performance of the solar cell.

As a method for ensuring the adhesive property with the sealing agent,there are known methods including (1) a method in which easy-adhesiontreatment is carried out on a surface of the rear surface protectionsheet that comes into contact with the sealing agent, and (2) a methodin which a film having a high adhesive property with the sealing agentis used on a surface of the rear surface protection sheet that comesinto contact with the sealing agent.

Examples of the above-described method (1) include surface treatmentsuch as corona treatment, and easy-adhesion coating treatment in whichan easy-adhesive is coated.

However, although the former method, i.e., the surface treatment such ascorona treatment can ensure the initial adhesive property, there is aproblem that the adhesive durability is poor.

Patent literatures 1, 5 and 6 disclose easy-adhesives that are used forthe latter method, i.e., the easy-adhesion coating treatment.

Patent literature 5 discloses a coating liquid containing across-linking agent selected from a group consisting of a polymercontaining an oxazoline group, a urea resin, a melamine resin, and anepoxy resin, and a resin component other than the cross-linking agentselected from a polyester resin or an acrylic resin whose glasstransition point is 20-100° C. (see claims 2 and 3 of the patentliterature). More specifically, Patent literature 5 discloses an examplein which a coating liquid containing an epoxy resin and an acrylic resinis used (see Example 5 of the patent literature). However, the adhesivestrength with the EVA sheet in this example is about 10-20 N for 20 mmwidth (i.e., 7.5-15 N for 15 mm width) (see Table 2 of the patentliterature). Since the adhesive property between the sealing agent andthe rear surface protection sheet has a significant influence on thedeterioration in the output performance of the solar cell, the marketdemands a higher adhesive property and the reliability for the adhesiveproperty under stricter conditions. The adhesive strength of about 20 Nfor 20 mm width cannot satisfy such demands in the market. Although theadhesive strength is improved in Patent literature 1, the market demandseasy-adhesives having higher properties.

Patent literature 6 discloses such a configuration that anadhesion-improving layer in which at least one type of a resin selectedfrom a group consisting of a polyester-based resin and a polyesterpolyurethane-based resin is cross-linked by a cross-linking agentcomposed of alkylated melamine or polyisocyanate on a polyester film isprovided on a surface of a rear surface protection sheet that comes intocontact with a sealing agent.

As for the above-described method (2) (method in which a film having ahigh adhesive property with the sealing agent is used on a surface ofthe rear surface protection sheet that comes into contact with thesealing agent), Patent literature 7, for example, discloses a methodusing polybutylene terphthalate (PET).

However, since the film like this usually has a thickness of severaltens of micrometers, the cost becomes higher in comparison to theabove-described easy-adhesion treatment.

Further, Patent literature 8, which is laid open after the previousapplication from which the present application claims priority wasfiled, discloses a back sheet for a solar cell module in which anadhesive layer composed of an acryl-based adhesive containing an acrylicpolymer that is obtained by polymerizing a monomer component containinga monomer expressed by a general formula (I) shown below on a surfacethat is bonded to filler material (sealing agent) constituting the solarcell module is formed.<Chemical 1>CH₂═C(R¹)—CO—OZ  Formula (1)In the formula, R¹ represents a hydrogen atom or a methyl group, and Zrepresents a hydrocarbon group having a carbon number of 4-25.

Further, Patent literature 9 discloses a rear surface protection sheetof a solar cell element including: a primer layer composed of a fluoriccopolymer, an acrylic copolymer, or a polyurethane-based copolymer(polymer a); a polymeric monomer and/or an oligomer having at least oneethylene unsaturated group for photo-curing (monomer b); and/or acompound containing at least one ethylene unsaturated group and two ormore isocyanate groups in the molecule (polyisocyanate c).

CITATION LIST Patent Literature

-   Patent literature 1: Japanese Unexamined Patent Application    Publication No. 2009-246360-   Patent literature 2: Japanese Unexamined Patent Application    Publication No. 2004-200322-   Patent literature 3: Japanese Unexamined Patent Application    Publication No. 2004-223925-   Patent literature 4: Japanese Unexamined Patent Application    Publication No. 2001-119051-   Patent literature 5: Japanese Unexamined Patent Application    Publication No. 2006-152013-   Patent literature 6: Japanese Unexamined Patent Application    Publication No. 2007-136911-   Patent literature 7: Japanese Unexamined Patent Application    Publication No. 2010-114154-   Patent literature 8: Japanese Unexamined Patent Application    Publication No. 2010-263193-   Patent literature 9: Japanese Unexamined Patent Application    Publication No. 2011-18872

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide an easy-adhesive havingan excellent adhesive property and an excellent adhesive durability fora solar cell rear surface protection sheet, a solar cell rear surfaceprotection sheet, and a solar cell module formed by using the solar cellrear surface protection sheet.

Solution to Problem

The present invention relates to an easy-adhesive for a solar cell rearsurface protection sheet, containing:

a (meth)acrylic-based copolymer (A) having a glass transitiontemperature of 10-60° C., a number average molecular weight of25,000-250,000, and a hydroxyl value of 2-100 (mgKOH/g), and having onecarbon-carbon double bond for every 5-500 side chains; and

a polyisocyanate compound (B), the polyisocyanate compound (B) beingcontained in such an amount that a number of an isocyanate group is0.1-5 for every one hydroxyl group contained in the (meth)acrylic-basedcopolymer (A).

According to the present invention, the (meth)acrylic-based copolymer(A) is preferably a copolymer selected from a group consisting offollowing acrylic-based copolymers (A1) to (A4):

a (meth)acrylic-based copolymer (A1): a (meth)acrylic-based copolymerthat is formed by reacting a (meth)acrylic-based monomer having acarboxyl group (a3) with a glycidyl group of a side chain contained in acopolymer containing a (meth)acrylic-based monomer having a glycidylgroup (a1), a (meth)acrylic-based monomer having a hydroxyl group (a2),and a (meth)acrylic-based monomer having no glycidyl group, no hydroxylgroup, and no carboxyl group (a4) as constitutional units;

a (meth)acrylic-based copolymer (A2): a (meth)acrylic-based copolymerthat is formed by reacting a (meth)acrylic-based monomer having aglycidyl group (a1) with a carboxyl group contained in a copolymercontaining a (meth)acrylic-based monomer having a carboxyl group (a3), a(meth)acrylic-based monomer having a hydroxyl group (a2), and a(meth)acrylic-based monomer having no glycidyl group, no hydroxyl group,and no carboxyl group (a4) as constitutional units;

a (meth)acrylic-based copolymer (A3): a (meth)acrylic-based copolymerthat is formed by reacting a (meth)acrylic-based monomer having anisocyanate group (a5) with a part of a hydroxyl group contained in acopolymer containing a (meth)acrylic-based monomer having a hydroxylgroup (a2) and a (meth)acrylic-based monomer having no hydroxyl group(a6) as constitutional units; and

a (meth)acrylic-based copolymer (A4): a (meth)acrylic-based copolymerthat is formed by reacting a (meth)acrylic-based monomer having ahydroxyl group (a2) with an acid anhydride group contained in acopolymer containing a maleic anhydride, a (meth)acrylic-based monomerhaving a hydroxyl group (a2), and a (meth)acrylic-based monomer havingno hydroxyl group (a6) as constitutional units.

Further, the polyisocyanate compound (B) is preferably a blockedpolyisocyanate compound (B1).

Further, the present invention relates to a solar cell rear surfaceprotection sheet (V′) including: an uncured easy-adhesive layer (D′)formed by the above-described easy-adhesive for a solar cell rearsurface protection sheet; and a plastic film (E).

Further, the present invention relates to a solar cell module including:a solar battery cell (III); a solar cell front surface protectionmaterial (I) that protects the solar battery cell (III) through asealing agent (II) disposed on a light-receiving side, the solar cellfront surface protection material (I) being disposed on thelight-receiving side; and a solar cell rear surface protection sheet (V)that protects the solar battery cell (III) through a sealing agent (IV)disposed on a non-light-receiving side, the solar cell rear surfaceprotection sheet (V) being disposed on the non-light-receiving side, inwhich the solar cell rear surface protection sheet (V) is obtained bydisposing a solar cell rear surface protection sheet (V′) including aplastic film (E) and an uncured easy-adhesive layer (D′) formed by aneasy-adhesive for a solar cell rear surface protection sheet describedin one of the above-described aspects in such a manner that theeasy-adhesive layer (D′) comes into contact with the sealing agent (IV)disposed on the non-light-receiving side, and curing the easy-adhesivelayer (D′).

Further, the sealing agent (IV) disposed on the non-light-receiving sidepreferably contains an organic peroxide. Further, the sealing agent (IV)disposed on the non-light-receiving side preferably contains anethylene-vinyl acetate copolymer (EVA) as a main component.

Advantageous Effects of Invention

By using an easy-adhesive for a solar cell rear surface protection sheetaccording to the present invention, the present invention provides anadvantageous effect that an easy-adhesive having an excellent adhesiveproperty and an excellent adhesive durability for a solar cell rearsurface protection sheet, a solar cell rear surface protection sheet,and a solar cell module formed by using the solar cell rear surfaceprotection sheet can be provided. By using a solar cell rear surfaceprotection sheet according to the present invention, it is possible toprovide a solar cell module capable of minimizing the deterioration inthe output performance even when the solar cell module is exposed to ahigh-temperature and high-humidity environment for a long time.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically shows a cross section of a solar cell moduleaccording to the present invention.

DESCRIPTION OF EMBODIMENTS

The present invention is explained hereinafter in detail. Note thatneedless to say, other exemplary embodiments are also included in thescope of the present invention as long as they meet the purport of thepresent invention. Further, in this specification, when a range ofnumerical values are specified by using a symbol “−”, the values writtenin front and behind the symbol “−” are also included as the lower-limitvalue and the upper-limit value of the range. Further, in thisspecification, terms “film” and “sheet” are not differentiated accordingto the thickness. In other words, the term “sheet” in this specificationincludes films having a small thickness, and the term “film” in thisspecification includes sheets having a large thickness.

FIG. 1 is a schematic cross section of a solar cell module according tothe present invention. A solar cell module 100 includes at least a solarcell front surface protection material (I), a light-receiving-sidesealing agent (II), a solar battery cell (III), anon-light-receiving-side sealing agent (IV), and a solar cell rearsurface protection sheet (V). The light-receiving side of the solarbattery cell (III) is protected by the solar cell front surfaceprotection material (I) through the light-receiving-side sealing agent(II). On the other hand, the non-light-receiving side of the solarbattery cell (III) is protected by the solar cell rear surfaceprotection sheet (V) through the non-light-receiving-side sealing agent(IV). An easy-adhesive layer composed of an easy-adhesive for a solarcell rear surface protection sheet (hereinafter, also referred to as“easy-adhesive”) is laminated on the surface layer that comes intocontact with the solar cell rear surface protection sheet (V) on thenon-light-receiving side.

Note that the easy-adhesive layer (D′) that is formed by aneasy-adhesive for a solar cell rear surface protection sheet accordingto the present invention undergoes a cross-linking reaction by using athermocompression bonding process that is carried out when the solarcell module 100 is formed. In the present invention, an easy-adhesivelayer for which the thermocompression bonding process has not performedyet is referred to as “easy-adhesive layer (D′)”, and a cross-kinkedeasy-adhesive layer for which the thermocompression bonding process hasalready performed is referred to as “easy-adhesive layer (D)”. In thismanner they are differentiated from each other. Similarly, a solar cellrear surface protection sheet for which the thermocompression bondingprocess has not performed yet is referred to as “solar cell rear surfaceprotection sheet (V′)”, and a solar cell rear surface protection sheetfor which the thermocompression bonding process has already performed isreferred to as “solar cell rear surface protection sheet (V)”. In thismanner they are differentiated from each other.

A (meth)acrylic-based copolymer (A) contained, in an easy-adhesive for asolar cell rear surface protection sheet according to the presentinvention is explained.

The (meth)acrylic-based copolymer (A) has a glass transition temperatureof 10-60° C., a number average molecular weight of 25,000-250,000, and ahydroxyl value of 2-100 (mgKOH/g), and has one carbon-carbon double bondfor every 5-500 side chains.

If the (meth)acrylic-based copolymer (A) has a glass transitiontemperature higher than 60° C., the coating of the easy-adhesive becomesharder and thus the adhesive strength with the sealing agentdeteriorates. If it is lower than 10° C., tucks are generated on thesurface of the easy-adhesive coating. Therefore, when the solar cellrear surface protection sheet is rolled after being manufactured, ittends to cause blocking. More preferably, the glass transitiontemperature of the (meth)acrylic-based copolymer (A) is 20-50° C.

Note that the glass transition temperature in this specification means aglass transition temperature of a resin that is obtained by drying the(meth)acrylic-based copolymer (A) to a 100% solid-content state,measured by the differential scanning calorimetry (DSC). For example, toobtain the glass transition temperature, an aluminum pan containing asample that is obtained by weighing about 10 mg of the sample andanother aluminum pan containing no sample are set in a DSC apparatus.They are rapidly cooled to −50° C. in a nitrogen gas stream by usingliquid nitrogen. After that, they are heated to 100° C. at a rate of 20°C./minute. Then, a DSC curve is plotted. An extrapolation glasstransition start temperature (Tig) is obtained from an intersectionpoint between a straight line that is drawn by extending the baseline onthe low temperature side of the DSC curve (section of the DSC curve in atemperature range in which no transition and no reaction occur in thetest specimen) to the high temperature side and a tangential line thatis drawn through such points that the slope of the curve in thestep-like changing section of the glass transition is maximized. Then,this obtained extrapolation glass transition start temperature can bedetermined as the glass transition temperature. Values measured by theabove-described method are used as glass transition temperaturesaccording to the present invention.

If the number average molecular weight of the (meth)acrylic-basedcopolymer (A) is higher than 250,000, the adhesive strength with thesealing agent deteriorates, whereas if it is lower than 25,000, themoisture/heat resistance of the easy-adhesive coating deteriorates andthus the adhesive strength with the sealing agent deteriorates after themoisture/heat resistance test. The number average molecular weight ofthe (meth)acrylic-based copolymer (A) is preferably 25,000-150,000. Morepreferably, the number average molecular weight is 30,000-100,000. Morepreferably, the number average molecular weight is 30,000-75,000.Particularly preferably, the number average molecular weight is30,000-50,000.

Note that the above-described number average molecular weight is apolystyrene-converted value of the (meth)acrylic-based copolymer (A)obtained by the gel permeation chromatography (GPC). For example, it isa value obtained by using polystyrene as a standard sample under theconditions that: the temperature of the column(s) (KF-805L, KF-803L andKF-802 available from SHOWA DENKO K. K.) is 40° C.; THF is used as theeluant; the flow rate is 0.2 ml/min; the detection is carried out by RImeasurement; and the sample concentration is 0.02%. Values measured bythe above-described method are used as number average molecular weightsaccording to the present invention.

It is important that the hydroxyl value of the (meth)acrylic-basedcopolymer (A) is 2-100 mgKOH/g in terms of the solid content. Thehydroxyl value is preferably 2-50 mgKOH/g, and more preferably 2-30mgKOH/g. If the hydroxyl value of the (meth)acrylic-based copolymer (A)is higher than 100 mgKOH/g, the cross-linking of the easy-adhesivecoating becomes denser and thus the adhesive strength with the plasticfilm (E) deteriorates. Further, even if the adhesion could be maintainedin the early stage, the cross-linking reaction advances during themoisture/heat resistance test. As a result, there is a possibility thatthe adhesive strength deteriorates after the moisture/heat resistancetest. On the other hand, if it is lower than 2 mgKOH/g, thecross-linking of the easy-adhesive coating becomes sparser and thus themoisture/heat resistance of the coating deteriorates. Therefore, theadhesive strength with the sealing agent deteriorates after themoisture/heat resistance test.

The (meth)acrylic-based copolymer (A) used in the present invention hasone carbon-carbon double bond for every 5-500 side chains. Further, the(meth)acrylic-based copolymer (A) preferably has one carbon-carbondouble bond for every 5-300 side chains. If the rate at which the(meth)acrylic-based copolymer (A) has carbon-carbon double bonds ishigher than this range, the cross-linking reaction of the easy-adhesiveoccurs excessively and thus the adhesive strength deteriorates. On theother hand, if the rate is lower than this range, the cross-linkingreaction does not occur sufficiently and thus a satisfactory adhesivestrength cannot be obtained.

There are no particular restrictions on the sealing agent (II) locatedon the light-receiving side of the solar battery cell (III) and thesealing agent (IV) located on the non-light-receiving side. That is,publicly-known materials may be suitably used for them. Examples of thesuitable materials include an EVA (ethylene-vinyl acetate copolymer),polyvinyl butyral, polyurethane, and polyolefin. Among them, an EVA ismainly used in consideration of the cost. Although a sealing agent inthe form of a sheet (including the form of a film) may be used for thesealing agents (II) and (IV) with ease, a sealing agent in the form of apaste may be also used.

The sealing agent (II) located on the light-receiving side and thesealing agent (IV) located on the non-light-receiving side may containan organic peroxide(s). By containing an organic peroxide(s) in thesealing agents (II) and (IV), it is possible, when the solar batterycell (III) is sandwiched between the sealing agents (II) and (IV) andheated, to cross-link the sealing agent (II), to cross-link the sealingagent (II) with the sealing agent (IV), and to cross-link the sealingagent (IV) by a radical reaction with high efficiency.

It is believed that by containing an organic peroxide(s) in thenon-light-receiving-side sealing agent (IV), when the heating andsealing are performed, the organic peroxide(s) also acts on thecarbon-carbon double bonds contained in the uncured easy-adhesive layer(D′) (i.e., easy-adhesive layer (D′) for which the curing process hasnot performed yet). As a result, it is believed that thenon-light-receiving-side sealing agent (IV) is cross-linked with theuncured easy-adhesive layer (D′) and the uncured easy adhesive layer(D′) is cross-linked.

Therefore, in the present invention, the term “carbon-carbon doublebonds” means carbon-carbon double bond parts (C═C) that are active to aradical reaction and can be mutually polymerized. Therefore,carbon-carbon double bonds that are inactive to the reaction, such asthe benzene ring and the pyridine ring, are not included in thecarbon-carbon double bonds in the present invention. Among them, it ispreferable to contain a carbon-carbon double bond having a highreactivity such as a (meth)acryloyl group. Note that in thisspecification, the term “curing process” means a process for joining thesealing agent (IV) with the solar cell rear surface protection sheet (V)after they are placed on top of each other.

Examples of the (meth)acrylic-based copolymer (A) like this include thefollowing (meth)acrylic-based copolymers (A1) to (A4).

A (meth)acrylic-based copolymer (A1) is a (meth)acrylic-based copolymerthat is formed by reacting a (meth)acrylic-based monomer having acarboxyl group (a3) with glycidyl groups of side chains contained in acopolymer containing a (meth)acrylic-based monomer having a glycidylgroup (a1), a (meth)acrylic-based monomer having a hydroxyl group (a2),and a (meth)acrylic-based monomer having no glycidyl group, no hydroxylgroup, and no carboxyl group (a4) as constitutional units.

That is, a copolymer containing a (meth)acrylic-based monomer having aglycidyl group (a1), a (meth)acrylic-based monomer having a hydroxylgroup (a2), and a (meth)acrylic-based monomer having no glycidyl group,no hydroxyl group, and no carboxyl group (a4) as constitutional units isobtained. Then, side chains of carbon-carbon double bonds can beintroduced by reacting the (meth)acrylic-based monomer having a carboxylgroup (a3) with all or part of glycidyl groups of side chains containedin the copolymer and thereby using the glycidyl groups as startingpoints.

A (meth)acrylic-based copolymer (A2) is a copolymer that is obtained byintroducing side chains of carbon-carbon double bonds by using carboxylgroups as starting points. That is, the (meth)acrylic-based copolymer(A2) is a (meth)acrylic-based copolymer that is formed by reacting a(meth)acrylic-based monomer having a glycidyl group (a1) with carboxylgroups contained in a copolymer containing a (meth)acrylic-based monomerhaving a carboxyl group (a3), a (meth)acrylic-based monomer having ahydroxyl group (a2), and a (meth)acrylic-based monomer having noglycidyl group, no hydroxyl group, and no carboxyl group (a4) asconstitutional units.

Similarly to the case of the (meth)acrylic-based copolymer (A1), it ispossible to react the glycidyl groups with all or part of the carboxylgroups in order to introduce side chains of carbon-carbon double bonds.

A (meth)acrylic-based copolymer (A3) is a (meth)acrylic-based copolymerthat is formed by reacting a (meth)acrylic-based monomer having anisocyanate group (a5) with a part of hydroxyl groups contained in acopolymer containing a (meth)acrylic-based monomer having a hydroxylgroup (a2) and a (meth)acrylic-based monomer having no hydroxyl group(a6) as constitutional units.

A (meth)acrylic-based copolymer (A4) is a (meth)acrylic-based copolymerthat is formed by reacting a (meth)acrylic-based monomer having ahydroxyl group (a2) with acid anhydride groups contained in a copolymercontaining a maleic anhydride, a (meth)acrylic-based monomer having ahydroxyl group (a2), and a (meth)acrylic-based monomer having nohydroxyl group (a6) as constitutional units.

Examples of the (meth)acrylic-based monomer having a glycidyl group (a1)include glycidyl acrylate, glycidyl methacrylate, and 4-hydroxybutylacrylate glycidyl ether.

Examples of the (meth)acrylic-based monomer having a hydroxyl group (a2)include 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, and4-hydroxybutyl(meth)acrylate. A hydroxyl group derived from this(meth)acrylic-based monomer having a hydroxyl group (a2) has a functionof reacting with a polyisocyanate compound (B) (which is describedlater) and thereby forming the easy-adhesive layer (D) that is a curedsubstance of the uncured easy-adhesive layer (D′).

Further, in the present invention, a monomer other than the(meth)acrylic-based monomer having a hydroxyl group (a2) is defined as a(meth)acrylic-based monomer having no hydroxyl group (a6).

Examples of the (meth)acrylic-based monomer having a carboxyl group (a3)include an acrylic acid, a methacrylic acid, a crotonic acid, anitaconic acid, and a citraconic acid.

A monomer other than the above-mentioned (meth)acrylic-based monomerhaving a glycidyl group (a1), the (meth)acrylic-based monomer having ahydroxyl group (a2), and the (meth)acrylic-based monomer having acarboxyl group (a3) is defined as a (meth)acrylic-based monomer havingno glycidyl group, no hydroxyl group, and no carboxyl group (a4).

Examples of the (meth)acrylic-based monomer having no glycidyl group, nohydroxyl group, and no carboxyl group (a4) include methyl(meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, iso-butyl(meth)acrylate, tert-butyl (meth)acrylate, 2-ethylhexyl(meth)acrylate,and octyl (meth)acrylate.

Examples of the (meth)acrylic-based monomer having an isocyanate group(a5) include 2-isocyanato ethyl acrylate and 2-isocyanato ethylmethacrylate. Examples of products of these substances include KarenzAOI and Karenz MOI available from SHOWA DENKO K. K.

In addition to the (meth)acrylic-based monomers (a1) to (a6), vinylacetate, vinyl ether, vinyl propionate, styrene, and the like may bealso used as appropriate for the formation of the (meth)acrylic-basedcopolymers (A1) to (A4).

Incidentally, the above-described (meth)acrylic-based monomers (a1) to(a6) can be expressed in a general formula “CH₂═CR¹—CO—OR²”.

In the formula, R¹ represents a hydrogen atom or a methyl group.

R² represents a monovalent substituent having a functional group uniqueto each monomer such as a carboxyl group, a glycidyl group, ahydroxyalkyl group, and an isocyanatoalkyl group.

In the case of the (meth)acrylic-based monomers (a1) to (a6), the R²part is regarded as a side chain as opposed to the main chain formed bythe CH₂═CR¹ polymerization. Therefore, the R² part is counted as oneside chain.

Further, in the case of monomers that are not expressed by theabove-mentioned general formula such as styrene and maleic anhydride,the part other than the part that forms a carbon-carbon double bond inthe polymerization and thereby forms the main chain of the copolymer isreferred to as a side chain. Therefore, it is counted in such a mannerthat one monomer has one side chain.

For example, supposing that maleic anhydride is reacted with2-hydroxyethyl acrylate, the anhydrous ring of the maleic anhydrideopens and thereby generates a carboxyl group and an ester linkage part.Even in the case like this, they are collectively counted as one sidechain.

When the side chain is defined as described above, the ratio of sidechains having a carbon-carbon double bond to all the side chains can becalculated in the following manner.

For example, the mole ratio of the monomers constituting a copolymerthat is copolymerized in a weight ratio “MMA (methyl methacrylate,molecular weight 100)/n-BMA (n-butyl methacrylate, molecular weight142)/HEMA (2-hydroxyethyl methacrylate, molecular weight 130)/GMA(glycidyl methacrylate, molecular weight 142)=18/78/2/2” is expressed as“MMA/n-BMA/HEMA/GMA=23.7/72.4/2/1.9”. The mole ratio is equal to theratio of the number of monomers. Further, because of the above-describeddefinition of the side chain, it is counted in such a manner that eachmonomer has one side chain. Therefore, the mole ratio is equal to theratio of the number of side chains. Therefore, this copolymer contains1.9 glycidyl groups for every 100 side chains.

Further, the (meth)acrylic-based copolymer (A) that is obtained bydenaturing the glycidyl groups contained in the above-describedcopolymer by an equal mole quantity of an acrylic acid contains sidechains that are obtained in which the 1.9 glycidyl groups for every 100side chains are converted into an equal number of carbon-carbon doublebonds. That is, it can be safely said that the (meth)acrylic-basedcopolymer (A) contains one carbon-carbon double bond for every 53 sidechains.

The first stage of the formation of the (meth)acrylic-based copolymer(A1), i.e., the stage in which (meth)acrylic-based monomers (a1), (a2)and (a4) are polymerized, the first stage of the formation of the(meth)acrylic-based copolymer (A2), i.e., the stage in which(meth)acrylic-based monomers (a3), (a2) and (a4) are polymerized, thefirst stage of the formation of the (meth)acrylic-based copolymer (A3),i.e., the stage in which (meth)acrylic-based monomers (a2) and (a6) arepolymerized, and the first stage of the formation of the(meth)acrylic-based copolymer (A4), i.e., the stage in which maleicanhydride and (meth)acrylic-based monomers (a2) and (a6) are polymerizedcan be carried out by an ordinary radical polymerization reaction. Thereare no restrictions on the reaction method. That is, it can be carriedout by using any publicly-known polymerization method such as solutionpolymerization, mass polymerization, and emulsion polymerization.However, the solution polymerization is preferred because the control ofthe reaction is easy and the subsequent operation can be immediatelyperformed.

Examples of the solvent include methyl ethyl ketone, methyl isobutylketone, toluene, cellosolve, ethyl acetate, and butyl acetate. That is,any solvent in which the resin according to the present invention can bedissolved can be used without any restrictions. Further, only one typeof a solvent may be used, or two or more solvents may be mixed. Further,as for the polymerization initiator used in the polymerization reaction,publicly-known initiators including: organic peroxides such as benzoylperoxide, acetyl peroxide, methyl ethyl ketone peroxide, and lauroylperoxide; and azo-based initiators such as azobisisobutyronitrile may beused. That is, there are no particular restrictions on thepolymerization initiator. Further, for each of the (meth)acrylic-basedcopolymers (A1) to (A4), only one type of a substance may be used, forexample, for the (meth)acrylic-based monomer (a2), or two or more typesof compounds may be used together. This is also true for the(meth)acrylic-based monomers (a1), (a3), (a4), (a5) and (a6).

Next, the polyisocyanate compound (B) is explained.

The polyisocyanate compound (B) reacts with hydroxyl groups that arederived from the (meth)acrylic-based monomer having a hydroxyl group(a2) and contained in the (meth)acrylic-based copolymer (A), and therebygives a moisture/heat resistance to the easy-adhesive layer. Inaddition, the polyisocyanate compound (B) can also improve thecontacting property with the plastic film (E) constituting the rearsurface protection sheet and with the sealing agent such as an EVA whichis used as the non-light-receiving-side sealing agent (IV). Therefore,it is important that the polyisocyanate compound (B) contains at leasttwo isocyanate groups in one molecule. Examples of the polyisocyanatecompound (B) include aromatic polyisocyanate, aliphatic chainpolyisocyanate, and alicyclic polyisocyanate. Only one type of acompound may be used for the polyisocyanate compound (B), or two or moretypes of compounds may be used together.

Examples of the aromatic polyisocyanate include 1,3-phenylenediisocyanate, 4,4′-diphenyl diisocyanate, 1,4-phenylene diisocyanate,4,4′-diphenylmethan diisocyanate, 2,4-tolylene diisocyanate,2,6-tolylene diisocyanate, 4,4′-toluidine diisocyanate,2,4,6-triisocyanate toluene, 1,3,5-triisocyanate benzene, dianisidinediisocyanate, 4,4′-diphenyl ether diisocyanate, 4,4′,4″-triphenylmethane triisocyanate.

Examples of the aliphatic chain polyisocyanate include trimethylenediisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate(HDI), pentamethylene diisocyanate, 1,2-propylene diisocyanate,2,3-butylene diisocyanate, 1,3-butylene diisocyanate, dodecamethylenediisocyanate, and 2,4,4-trimethyl hexamethylene diisocyanate.

Examples of the alicyclic polyisocyanate include 3-isocyanatemethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI), 1,3-cyclopentanediisocyanate, 1,3-cyclohexane diisocyanate, 1,4-cyclohexanediisocyanate, methyl-2,4-cyclohexane diisocyanate,methyl-2,6-cyclohexane diisocyanate, 4,4′-methylenebis(cyclohexylisocyanate), and 1,4-bis(isocyanatemethyl)cyclohexane.

Further, in addition to the above-mentioned polyisocyanate, examplesalso include an adduct of the above-mentioned polyisocyanate and apolyol compound such as trimethylolpropane, a biuret and an isocyanurateof the above-mentioned polyisocyanate, and an adduct of theabove-mentioned polyisocyanate and publicly-known polyether polyol,polyester polyol, acryl polyol, polybutadiene polyol, polyisoprenepolyol, or the like.

Among these polyisocyanate compounds (B), a low-yellowing type aliphaticor alicyclic polyisocyanate is preferred in terms of the design.Further, an isocyanurate is preferred in terms of the moisture/heatresistance. More specifically, an isocyanurate of hexamethylenediisocyanate (HDI) and an isocyanurate 3-isocyanatemethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI) are preferred.

Further, it is possible to obtain a blocked polyisocyanate compound (B1)by reacting roughly the whole amount of the isocyanate groups of thesepolyisocyanate compounds (B) with a blocking agent. The uncuredeasy-adhesive layer (D′) obtained by applying an easy-adhesive for asolar cell rear surface protection sheet according to the presentinvention is preferably in an un-cross-linked state before it is bondedto the sealing agent (IV) to manufacture a solar cell module. Therefore,the polyisocyanate compound (B) is preferably a blocked polyisocyanatecompound (B1).

Examples of the blocking agent include: phenols such as phenol,thiophenol, methylthiophenol, xylenol, cresol, resorcinol, nitrophenol,and chlorophenol; oximes such as acetone oxime, methyl ethyl ketoneoxime, and cyclohexanone oxime; alcohols such as methanol, ethanol,n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol,t-butyl alcohol, t-pentanol, ethylene glycol monomethyl ether, ethyleneglycol monobutyl ether, diethylene glycol monomethyl ether, propyleneglycol monomethyl ether, and benzyl alcohol; pyrazoles such as3,5-dimethylpyrazole and 1,2-pyrazole; triazoles such as 1,2,4-triazole;halogen-substituted alcohols such as ethylenechlorohydrin and1,3-dichloro-2-propanol; lactams such as ε-caprolactam, δ-valerolactam,γ-butyrolactam, and β-propllactam; and active methylene compounds suchas methyl acetoacetate, ethyl acetoacetate, acetylacetone, methylmalonate, and ethyl malonate. In addition, examples also include amines,imides, mercaptans, imines, ureas, and diaryls. Only one type of asubstance may be used for the blocking agent, or two or more types ofsubstances may be used together for the blocking agent.

Among these blocking agents, those having a dissociation temperature of80-150° C. are preferred. If the dissociation temperature is lower than80° C., the curing reaction advances when the easy-adhesive is appliedand the solvent is vaporized. As a result, there is a possibility thatthe contacting property with the filler deteriorates. If thedissociation temperature is higher than 150° C., the curing reactiondoes not advance sufficiently in the vacuum thermocompression bondingprocess that is performed when a solar cell module is formed. As aresult, the contacting property with the filler deteriorates.

Examples of the blocking agent having a dissociation temperature of80-150° C. include methyl ethyl ketone oxime (dissociation temperature:140° C., the same applies hereafter), 3,5-dimethylpyrazole (12° C.) anddiisopropylamine (120° C.).

It is necessary that the polyisocyanate compound (B) in theeasy-adhesive according to the present invention is used in such anamount that 0.1-5 isocyanate groups are present for every one hydroxylgroup of the (meth)acrylic-based copolymer (A). Preferably, 0.5-4isocyanate groups are present for every one hydroxyl group. If thenumber is less than 0.1, the cross-link density becomes so low that themoisture/heat resistance becomes unsatisfactory. If the number isgreater than 5, excessive isocyanate reacts with moisture in theatmosphere during the moisture/heat resistance test. As a result, theeasy-adhesive coating becomes harder, and thereby causing adeterioration in the adhesion strength with the plastic film (E)constituting the rear surface protection sheet and/or with the sealingagent such as an EVA which is used as the non-light-receiving-sidesealing agent (IV).

The easy-adhesive according to the present invention may also containorganic particles or inorganic particles in an amount of 0.01-30 wt.pts. based on 100 wt. pts. of the solid content. More preferably, theorganic particles or inorganic particles may be contained in an amountof 0.1-10 wt. pts. By containing these particles in the easy-adhesive,it is possible to reduce the tucks on the surface of the uncuredeasy-adhesive layer (D′). If the content is less than 0.01 wt. pts., thetucks on the surface of the uncured easy-adhesive layer (D′) cannot besufficiently reduced. On the other hand, if the amount of theabove-mentioned various particles is excessively high, they impair theclosely-contact between the uncured easy-adhesive layer (D′) and thesealing agent, and thereby possibly causing a deterioration in theadhesive strength.

In particular, as for the organic particles, it is preferable to useorganic particles whose melting point or softening point is equal to orhigher than 150° C. If the melting point or softening point is lowerthan 150° C. there is a possibility that the particles are softened inthe vacuum thermocompression bonding process that is performed when asolar cell module is formed, and thereby impairing the adhesion with thesealing agent.

Specific examples of the organic particles include polymer particlesmade of polymers such as a polymethyl methacrylate resin, a polystyreneresin, a nylon resin (registered trademark), a melamine resin, aguanamine resin, a phenol resin, a urea resin, a silicon resin, amethacrylate resin, and an acrylate resin. Examples also includecellulose powder, nitrocellulose powder, wood flour, used-paper powder,chaff powder, and starch. Only one type of a substance may be used forthe organic particles, or two or more types of substances may be usedtogether for the organic particles.

The above-mentioned polymer particles can be obtained by apolymerization method such as the emulsion polymerization method, thesuspension polymerization method, the dispersion polymerization method,the soap-free polymerization method, the seed polymerization method, andthe micro-suspension polymerization method. Further, the above-describedorganic particles may contain impurities in such an extent that theircharacteristics are not impaired. Further, the particles may have anyshape or form including a powder form, a particle form, a granular form,a plate form, and a fiber form.

Examples of the inorganic particles include inorganic particlescontaining an oxide, a hydroxide, a sulfate, a carbonate, a silicate, orthe like of a metal such as magnesium, calcium, barium, zinc, zirconium,molybdenum, silicon, antimony, and titanium. More specifically, examplesinclude inorganic particles containing silica gel, aluminum oxide,calcium hydroxide, calcium carbonate, magnesium oxide, magnesiumhydroxide, magnesium carbonate, zinc oxide, lead oxide, diatomaceousearth, zeolite, aluminosilicate, talc, white carbon, mica, glass fibers,glass powder, glass beads, clay, wollastonite, iron oxide, antimonyoxide, titanium oxide, lithopone, pumice powder, aluminum sulfate,zirconium silicate, barium carbonate, dolomite, molybdenum disulfide,iron sand, carbon black, or the like. Only one type of a substance maybe used for the inorganic particles, or two or more types of substancesmay be used together for the inorganic particles.

Further, the above-described inorganic particles may contain impuritiesin such an extent that their characteristics are not impaired. Further,the particles may have any shape or form including a powder form, aparticle form, a granular form, a plate form, and a fiber form.

Further, if necessary, a cross-linking accelerating agent may be addedin the easy-adhesive according to the present invention in such anextent that the advantageous effect of the present invention is notimpaired. The cross-linking accelerating agent acts as a catalyst thataccelerates the urethane bonding reaction by a hydroxyl group of the(meth)acrylic-based copolymer (A) and an isocyanate group of thepolyisocyanate compound (B). Examples of the cross-linking acceleratingagent include tin compounds, metal salts, and bases. Specific examplesof the cross-linking accelerating agent include tin octylate, dibutyltindiacetate, dibutyltin dilaurate, dioctyltin dilaurate, tin chloride,iron octylate, cobalt octylate, zinc naphthenate, triethylamine, andtriethylenediamine. Only one substance from these substances may beused, or two or more substances may be combined.

Further, if necessary, various additives such as a filler, a thixotropyimparting agent, an aging preventive agent, an antioxidant, anantistatic agent, a flame retardant, a heat conductive improver, aplasticizer, a dripping preventive agent, an antifoulant, an antiseptic,a bactericide, an antifoaming agent, a leveling agent, a curing agent, athickener, a pigment dispersing agent, and a silane coupling agent maybe added to the easy-adhesive according to the present invention in suchan extent that the advantageous effect of the present invention is notimpaired.

The easy-adhesive used in the present invention includes a solvent.

As for the solvent, it is possible to use, according to the compositionof the resin composition, an appropriate solvent(s) selected from:

alcohols such as methanol, ethanol, propanol, butanol, ethylene glycolmethyl ether, and diethylene glycol methyl ether;

ketones such as methyl ethyl ketone, methyl isobutyl ketone, andcyclohexanone;

ethers such as tetrahydrofuran, dioxane, ethylene glycol dimethyl ether,and diethylene glycol dimethyl ether;

hydrocarbons such as hexane, heptanes, and octane;

aromatic compounds such as benzene, toluene, xylene, and cumene;

esters such as ethyl acetate and butyl acetate; and the like. However, asolvent having a boiling point of 50-200° C. can be preferably used. Ifthe boiling point is lower than 50° C., the solvent tends to volatilizewhen the easy-adhesive is applied. As a result, the solid contentincreases, making it difficult to apply the easy-adhesive in a uniformthickness. If the boiling point is higher than 200° C., it becomesdifficult to dry up the solvent. Note that two or more solvents may beused together.

The easy-adhesive according to the present invention makes it possibleto form an easy-adhesive layer (D′) by applying it on a plastic film(E), and thereby to manufacture a Solar cell rear surface protectionsheet (V′) having an excellent adhesive property with a sealing agent(IV).

The coating of the easy-adhesive according to the present invention ontothe plastic film (E) may be carried out by using a publicly-knownmethod. Specific examples of the coating method include comma coating,gravure coating, reverse coating, roll coating, lip coating, and spraycoating. By applying the easy-adhesive by using these methods and thenvaporizing the solvent by a heating/drying process, an uncuredeasy-adhesive layer (D′) can be formed.

The thickness of the formed uncured easy-adhesive layer (D′) ispreferably 0.01-30 μm, and more preferably 0.1-10 μm.

As for the plastic film (E), a polyester-based resin film made of apolyester resin such as polyethylene terephthalate, polybutyleneterephthalate, and polyethylene naphthalate; an olefin film made ofpolyolefin such as polyethylene, polypropylene, and polycyclopentadiene;a fluorine-based film such as a polyvinyl fluoride film, apolyvinylidene fluoride film, a polytetrafluoroethylene film, and anethylene-tetrafluoroethylene copolymer film; an acryl film; a triacetylcellulose film; or the like may be used. It is preferable to use apolyester-based resin film in terms of the film rigidity and the cost.Further, among the polyester-based resin films, a polyethyleneterephthalate film is particularly preferred. The plastic film (E) maybe formed in a single layer or in a multi-layer structure having two ormore layers. Further, a vapor-deposition film(s) obtained byvapor-depositing a metal oxide or a nonmetal inorganic oxide may belaminated on the plastic film (E).

As for the vapor-deposited metal oxide or nonmetal inorganic oxide, anoxide of, for example, silicon, aluminum, magnesium, calcium, potassium,tin, sodium, boron, titanium, lead, zirconium, yttrium, or the like maybe used. Further, a fluoride of an alkali metal, an alkaline-earthmetal, or the like may be also used. Further, only one substance fromthese substances may be used, or two or more substances may be combined.

The metal oxide or nonmetal inorganic oxide may be deposited by using apublicly-known PVD method such as vacuum deposition, ion plating, andsputtering, or a CVD method such as plasma CVD and microwave CVD.

The plastic film (E) may be colorless or may contain a coloringcomponents) such as a pigment and a dye. Examples of the method forincorporating a coloring component(s) into the plastic film (E) includea method in which a coloring component(s) is kneaded into the film inadvance when the film is produced, and a method for printing a colorcomponent(s) on a colorless transparent film substrate. Further, a colorfilm and a colorless transparent film may be bonded together and used inthe bonded state.

In the solar cell rear surface protection sheet (V′), a single layer ora plurality of layers of a film layer(s) and/or a coating layer(s)formed from a metal foil (F) and/or a weatherproof resin layer (G) maybe provided on the surface of the plastic film (E) on which theeasy-adhesive layer (D′) is not formed.

As for the metal foil (F), an aluminum foil, an iron foil, a zinclaminated plate, or the like may be used. Among these foils/plates, analuminum foil is preferred in terms of the corrosion resistance. Thethickness is preferably 10-100 μm, and more preferably 20-50 μm. Variouspublicly-known adhesives may be used for the lamination of the metalfilm (F).

As for the weatherproof resin layer (G), a resin layer that is obtainedby laminating a polyvinylidene fluoride film, a polyester-based resinfilm made of a polyester-based resin such as polyethylene terephthalate,polybutylene terephthalate, and polynaphthalene terephthalate, or thelike by using various publicly-known adhesive may be used.Alternatively, a coating layer formed by applying a high weatherproofpaint such as Lumiflon available from Asahi Glass Co., Ltd, may be used.

Next, a solar cell module is explained.

The solar cell module 100 is obtained by laminating a solar cell frontsurface protection material (I), which is to be located on thelight-receiving side of a solar battery cell (III), on the solar batterycell (III) with an uncured sealing agent (II), which is to be located onthe light-receiving side of the solar battery cell (III), interposedtherebetween, laminating an uncured solar cell rear surface protectionsheet (V′) with an uncured sealing agent (IV), which is to be located onthe non-light-receiving side of the solar battery cell (III), interposedtherebetween, and performing a high-temperature thermocompressionbonding process under a reduced pressure.

There are no particular restrictions on the material for the solar cellfront surface protection material (I). However, preferred examples ofthe material include a glass plate and a plastic plate such as apolycarbonate plate and a polyacrylate plate. A glass plate is preferredin terms of the transparency, the weather resistance, and the strength.Further, among the glass plates, a white-plate glass having hightransparency is preferred.

The sealing agent such as an EVA which is used as the sealing agents(II) and (IV) may contain an additive(s) such as a UV absorber forimproving the weather resistance, a light stabilizer, an organicperoxide for cross-linking the EVA itself.

The easy-adhesive layer (D′) according to the present invention improvesthe adhesive strength with the sealing agent (IV) because carbon-carbondouble bonds are cross-linked in the high-temperature thermocompressionbonding process that is carried out when the solar cell module isformed. When an organic peroxide is contained in the sealing agent (IV),the cross-linking reaction is accelerated. As a result, an advantageouseffect of the present invention is maximized. Therefore, the sealingagent located on the non-light-receiving side preferably contains anorganic peroxide.

Examples of the solar battery cell (III) include a solar battery cell inwhich electrodes are provided on an optical/electrical conversion layermade of a compound semiconductor or the like, typified by crystallinesilicon, amorphous silicon, and copper indium selenide, and a solarbattery cell in which those optical/electrical conversion layer andelectrodes are laminated on a substrate made of glass or the like.

EXAMPLES

The present invention is explained hereinafter in a more detailed mannerby using examples. However, the present invention is not limited to theexamples shown below. Note that in the examples, “pts.” and “%”represent “wt. pts.” and “wt. %” respectively. Table 1 shows physicalproperties of (meth)acrylic-based copolymers.

<(Meth)Acrylic-Based Copolymer A1 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 18 pts. ofmethyl methacrylate, 78 pts. of n-butyl methacrylate, 2 pts. of2-hydroxyethyl methacrylate, 2 pts. of glycidyl methacrylate, and 100pts. of toluene, and the mixture was heated to 100° C. while beingstirred in a nitrogen atmosphere. Next, 0.15 pts. ofazobisisobutyronitrile was added and the mixture underwent apolymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone, 0.8 pts. of dimethylbenzylamine, and 1 pt. of an acrylic acid were added and the mixture washeated and stirred for 15 hours at 100° C. After it was confirmed thatthe acid value was equal to or less than 2, a (meth)acrylic-basedcopolymer A1 solution having a number average molecular weight of39,000, a hydroxyl value of 17.2 (mg KOH/g), and a Tg of 30° C. wasobtained. Further, the ratio of side chains containing a carbon-carbondouble bond was 1/53 and the solid content was 50%.

Note that the number average molecular weight, the hydroxyl value, theglass transition temperature, the acid value, and the hydroxyl valuewere measured in the below-described manner. Further, the ratio of sidechains containing a carbon-carbon double bond is a theoretical valuethat is obtained based on the amount of glycidyl methacrylate containedin the monomer used in the copolymerization.

<Measurement of Number Average Molecular Weight (Mn)>

Mn was obtained by using the above-described GPC (gel permeationchromatography).

<Measurement of Glass Transition Temperature (Tg)>

The glass transition temperature was obtained by the above-describeddifferential scanning calorimetric measurement (DSC) method.

Note that samples that were obtained by heating the above-describedacrylic resin solution for 15 minutes at 150° C. and thereby drying andsolidifying the acrylic resin solution were used as samples formeasuring the Tg.

<Measurement of Acid Value (AV)>

About 1 g of the sample (resin solution: about 50%) was preciselyweighed and put into a stoppered conical flask and 100 ml of atoluene/ethanol mixture solution (volume ratio: toluene/ethanol=2/1) wasadded, and the sample was dissolved in the mixture solution. Aphenolphthalein reagent was added in the solution as an indicator, andthe solution was left as it was for 30 seconds. After that, the solutionwas titrated with a 0.1 N alcoholic potassium hydroxide solution untilthe solution exhibited salmon-pink. The acid value was calculated by thefollowing expression. A value obtained when the resin was in a driedstate was used as the acid value (unit: mg KOH/g).Acid Value(mg KOH/g)={(5.611×a×F)/S}/(Concentration of NonvolatileContent/100)

where: S: amount of collected sample (g);

a: amount of consumed 0.1 N alcoholic potassium hydroxide solution (ml);and

F: titer of 0.1 N alcoholic potassium hydroxide solution.

<Measurement of Hydroxyl Value (OHV)>

About 1 g of the sample (resin solution: about 50%) was preciselyweighed and put into a stoppered conical flask and 100 ml of atoluene/ethanol mixture solution (volume ratio: toluene/ethanol=2/1) wasadded, and the sample was dissolved in the mixture solution. An exactly5 ml of an acetylating agent (solution obtained by dissolving 25 g ofacetic anhydride into pyridine of such an amount that the total volumebecame 100 ml) was added to the solution, and the mixture was stirredfor about one hour. A phenolphthalein reagent was added in the solutionas an indicator, and the solution was left as it was for 30 seconds.After that, the solution was titrated with a 0.1 N alcoholic potassiumhydroxide solution until the solution exhibited salmon-pink. Thehydroxyl value was calculated by the following expression. A valueobtained when the resin was in a dried state was used as the hydroxylvalue (unit: mg KOH/g).Hydroxyl Value(mg KOH/g)=[{(b-a)×F×28.25}/S]/(Concentration ofNonvolatile Content/100)+D

where: S: amount of collected sample (g);

a: amount of consumed 0.1 N alcoholic potassium hydroxide solution (ml);

b: amount of 0.1 N alcoholic potassium hydroxide solution consumed in ablank test (ml);

F: titer of 0.1 N alcoholic potassium hydroxide solution; and

D: acid value (mg KOH/g).

<(Meth)Acrylic-Based Copolymer A2 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 18 pts. ofmethyl methacrylate, 78 pts. of n-butyl methacrylate, 2 pts. of2-hydroxyethyl methacrylate, 2 pts. of glycidyl methacrylate, and 100pts. of toluene, and the mixture was heated to 100° C. while beingstirred in a nitrogen atmosphere. Then, 0.3 pts. ofazobisisobutyronitrile was added and the mixture underwent apolymerization reaction for 2 hours. Next, 0.05 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.05 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone, 0.8 pts. of dimethylbenzylamine, and 1 pt. of an acrylic acid were added and the mixture washeated and stirred for 15 hours at 100° C. After it was confirmed thatthe acid value was equal to or less than 2, a (meth)acrylic-basedcopolymer A2 solution having a number average molecular weight of26,000, a hydroxyl value of 16.5 (mg KOH/g), and a Tg of 30° C. wasobtained. Further, the ratio of side chains containing a carbon-carbondouble bond was 1/53 and the solid content was 50%.

<(Meth)Acrylic-Based Copolymer A3 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 18 pts. ofmethyl methacrylate, 78 pts. of n-butyl methacrylate, 2 pts. of2-hydroxyethyl methacrylate, 2 pts. of glycidyl methacrylate, and 100pts. of toluene, and the mixture was heated to 80° C. while beingstirred in a nitrogen atmosphere. Then, 0.075 pts. ofazobisisobutyronitrile was added and the mixture underwent apolymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone, 0.8 pts. of dimethylbenzylamine, and 1 pt. of an acrylic acid were added and the mixture washeated and stirred for 15 hours at 100° C. After it was confirmed thatthe acid value was equal to or less than 2, a (meth)acrylic-basedcopolymer A3 solution having a number average molecular weight of75,000, a hydroxyl value of 18.0 (mg KOH/g), and a Tg of 30° C. wasobtained. Further, the ratio of side chains containing a carbon-carbondouble bond was 1/53 and the solid content was 50%.

<(Meth)Acrylic-Based Copolymer A4 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 18 pts. ofmethyl methacrylate, 78 pts. of n-butyl methacrylate, 4 pts. of2-hydroxyethyl methacrylate, and 100 pts. of toluene, and the mixturewas heated to 100° C. while being stirred in a nitrogen atmosphere.Then, 0.15 pts. of azobisisobutyronitrile was added and the mixtureunderwent a polymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone and 0.03 pts. dibutyltindilaurate were added, and a solution obtained by dissolving 1.7 pts. of2-isocyanato ethyl methacrylate into 1.7 pts. of methyl ethyl ketone wasdropped in the mixture over two hours while stirring the mixture at 40°C. After it was confirmed that the isocyanate peak (2260 cm⁻¹) haddisappeared by IR measurement, a (meth)acrylic-based copolymer A4solution having a number average molecular weight of 38,000, a hydroxylvalue of 8.6 (mg KOH/g), and a Tg of 30° C. was obtained. Further, theratio of side chains containing a carbon-carbon double bond was 1/49 andthe solid content was 50%.

<(Meth)Acrylic-Based Copolymer A5 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 18 pts. ofmethyl methacrylate, 78 pts. of n-butyl methacrylate, 4 pts. of2-hydroxyethyl methacrylate, and 100 pts. of toluene, and the mixturewas heated to 100° C. while being stirred in a nitrogen atmosphere.Then, 0.15 pts. of azobisisobutyronitrile was added and the mixtureunderwent a polymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone and 0.03 pts. of dibutyltindilaurate were added, and a solution obtained by dissolving 2.9 pts. of2-isocyanato ethyl methacrylate into 2.9 pts. of methyl ethyl ketone wasdropped in the mixture over two hours while stirring the mixture at 40°C. After it was confirmed that the isocyanate peak (2260 cm⁻¹) haddisappeared by IR measurement, a (meth)acrylic-based copolymer A5solution having a number average molecular weight of 33,000, a hydroxylvalue of 2.1 (mg KOH/g), and a Tg of 27° C. was obtained. Further, theratio of side chains containing a carbon-carbon double bond was 1/28 andthe solid content was 50%.

<(Meth)Acrylic-Based Copolymer A6 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 18 pts. ofmethyl methacrylate, 78 pts. of n-butyl methacrylate, 2 pts. of2-hydroxyethyl methacrylate, 2 pts. of phthalic anhydride, and 100 pts.of toluene, and the mixture was heated to 100° C. while being stirred ina nitrogen atmosphere. Then, 0.15 pts. of azobisisobutyronitrile wasadded and the mixture underwent a polymerization reaction for 2 hours.Next, 0.07 pts. of azobisisobutyronitrile was further added and themixture underwent a polymerization reaction for 2 hours. Then, 0.07 pts.of azobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that 0.85 pts. of 2-hydroxyethyl methacrylate and 0.5 pts. oftriethylamine were added and the mixture was heated and stirred for 7hours at 110° C. As a result, a (meth)acrylic-based copolymer A6solution having a number average molecular weight of 36,000, a hydroxylvalue of 8.5 (mg KOH/g), and a Tg of 30° C. was obtained. Further, theratio of side chains containing a carbon-carbon double bond was 1/37 andthe solid content was 50%.

<(Meth)Acrylic-Based Copolymer A7 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 96 pts. ofn-butyl methacrylate, 2 pts. of 2-hydroxyethyl methacrylate, 2 pts. ofglycidyl methacrylate, and 100 pts. of toluene, and the mixture washeated to 100° C. while being stirred in a nitrogen atmosphere. Then,0.15 pts. of azobisisobutyronitrile was added and the mixture underwenta polymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone, 0.8 pts. of dimethylbenzylamine, and 1 pt. of an acrylic acid were added and the mixture washeated and stirred for 15 hours at 100° C. After it was confirmed thatthe acid value was equal to or less than 2, a (meth)acrylic-basedcopolymer A7 solution having a number average molecular weight of38,000, a hydroxyl value of 17.0 (mg KOH/g), and a Tg of 20° C. wasobtained. Further, the ratio of side chains containing a carbon-carbondouble bond was 1/50 and the solid content was 50%.

<(Meth)Acrylic-Based Copolymer A8 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 41 pts. ofmethyl methacrylate, 55 pts. of n-butyl methacrylate, 2 pts. of2-hydroxyethyl methacrylate, 2 pts. of glycidyl methacrylate, and 100pts. of toluene, and the mixture was heated to 100° C. while beingstirred in a nitrogen atmosphere. Then, 0.15 pts. ofazobisisobutyronitrile was added and the mixture underwent apolymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone, 0.8 pts. of dimethylbenzylamine, and 1 pt. of an acrylic acid were added and the mixture washeated and stirred for 15 hours at 100° C. After it was confirmed thatthe acid value was equal to or less than 2, a (meth)acrylic-basedcopolymer A8 solution having a number average molecular weight of41,000, a hydroxyl value of 8.5 (mg KOH/g), and a Tg of 51° C. wasobtained. Further, the ratio of side chains containing a carbon-carbondouble bond was 1/59 and the solid content was 50%.

<(Meth)Acrylic-Based Copolymer A9 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 20 pts. ofmethyl methacrylate, 63 pts. of n-butyl methacrylate, 15 pts. of2-hydroxyethyl methacrylate, 2 pts. of glycidyl methacrylate, and 100pts. of toluene, and the mixture was heated to 100° C. while beingstirred in a nitrogen atmosphere. Then, 0.15 pts. ofazobisisobutyronitrile was added and the mixture underwent apolymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone, 0.8 pts. of dimethylbenzylamine, and 1 pt. of an acrylic acid were added and the mixture washeated and stirred for 15 hours at 100° C. After it was confirmed thatthe acid value was equal to or less than 2, a (meth)acrylic-basedcopolymer A9 solution having a number average molecular weight of45,000, a hydroxyl value of 73.0 (mg KOH/g), and a Tg of 40° C. wasobtained. Further, the ratio of side chains containing a carbon-carbondouble bond was 1/55 and the solid content was 50%.

<(Meth)Acrylic-Based Copolymer A10 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 45 pts. ofmethyl methacrylate, 3 pts. of n-butyl methacrylate, 2 pts. of2-hydroxyethyl methacrylate, 15 pts. of glycidyl methacrylate, and 100pts. of toluene, and the mixture was heated to 100° C. while beingstirred in a nitrogen atmosphere. Then, 0.15 pts. ofazobisisobutyronitrile was added and the mixture underwent apolymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone, 0.8 pts. of dimethylbenzylamine, and 7.5 pts. of an acrylic acid were added and the mixturewas heated and stirred for 15 hours at 100° C. After it was confirmedthat the acid value was equal to or less than 2, a (meth)acrylic-basedcopolymer A10 solution having a number average molecular weight of37,000, a hydroxyl value of 70.2 (mg KOH/g), and a Tg of 57° C. wasobtained. Further, the ratio of side chains containing a carbon-carbondouble bond was 1% and the solid content was 52%.

<(Meth)Acrylic-Based Copolymer A11 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 19.5 pts.of methyl methacrylate, 78 pts. of n-butyl methacrylate, 2 pts. of2-hydroxyethyl methacrylate, 0.5 pts. of glycidyl methacrylate, and 100pts. of toluene, and the mixture was heated to 100° C. while beingstirred in a nitrogen atmosphere. Then, 0.15 pts. ofazobisisobutyronitrile was added and the mixture underwent apolymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone, 0.8 pts. of dimethylbenzylamine, and 0.25 pts. of an acrylic acid were added and the mixturewas heated and stirred for 15 hours at 100° C. After it was confirmedthat the acid value was equal to or less than 2, a (meth)acrylic-basedcopolymer A11 solution having a number average molecular weight of35,000, a hydroxyl value of 10.5 (mg KOH/g), and a Tg of 34° C. wasobtained. Further, the ratio of side chains containing a carbon-carbondouble bond was 1/222 and the solid content was 50%.

<(Meth)Acrylic-Based Copolymer A12 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 18 pts. ofmethyl methacrylate, 78 pts. of n-butyl methacrylate, 4 pts. of2-hydroxyethyl methacrylate, and 100 pts. of ethyl acetate, and themixture was heated to 77° C. while being stirred in a nitrogenatmosphere. Then, 0.05 pts. of azobisisobutyronitrile was added and themixture underwent a polymerization reaction for 2 hours. Next, 22 pts.of ethyl acetate and 0.05 pts. of azobisisobutyronitrile were added andthe mixture underwent a polymerization reaction for 2 hours. Then, 22pts. of ethyl acetate and 0.05 pts. of azobisisobutyronitrile werefurther added and the mixture underwent a polymerization reaction for 2hours. After that, 36 pts. of ethyl acetate and 0.05 pts. ofazobisisobutyronitrile were added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.05 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone and 0.03 pts. of dibutyltindilaurate were added, and a solution obtained by dissolving 1.7 pts. of2-isocyanato ethyl methacrylate into 1.7 pts. of methyl ethyl ketone wasdropped in the mixture over two hours while stirring the mixture at 40°C. After it was confirmed that the isocyanate peak (2260 cm⁻¹) haddisappeared by IR measurement, a (meth)acrylic-based copolymer A12solution having a number average molecular weight of 242,000, a hydroxylvalue of 8.0 (mg KOH/g), and a Tg of 30° C. was obtained. Further, theratio of side chains containing a carbon-carbon double bond was 1/49 andthe solid content was 35%.

<(Meth)Acrylic-Based Copolymer A13 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 20 pts. ofmethyl methacrylate, 55 pts. of n-butyl methacrylate, 25 pts. of2-hydroxyethyl methacrylate, and 100 pts. of toluene, and the mixturewas heated to 100° C. while being stirred in a nitrogen atmosphere.Then, 0.13 pts. of azobisisobutyronitrile was added and the mixtureunderwent a polymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone and 0.03 pts. of dibutyltindilaurate were added, and a solution obtained by dissolving 2.9 pts. of2-isocyanato ethyl methacrylate into 2.9 pts. of methyl ethyl ketone wasdropped in the mixture over two hours while stirring the mixture at 40°C. After it was confirmed that the isocyanate peak (2260 cm⁻¹) haddisappeared by IR measurement, a (meth)acrylic-based copolymer A13solution having a number average molecular weight of 44000, a hydroxylvalue of 98.7 (mg KOH/g), and a Tg of 42° C. was obtained. Further, theratio of side chains containing a carbon-carbon double bond was 1/51 andthe solid content was 50%.

<(Meth)Acrylic-Based Copolymer A14 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 18 pts. ofmethyl methacrylate, 80 pts. of n-butyl methacrylate, 2 pts. of2-hydroxyethyl methacrylate, and 100 pts. of toluene, and the mixturewas heated to 100° C. while being stirred in a nitrogen atmosphere.Then, 0.15 pts. of azobisisobutyronitrile was added and the mixtureunderwent a polymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. A (meth)acrylic-based copolymer A14solution having a number average molecular weight of 38,000, a hydroxylvalue of 8.6 (mg KOH/g), and a Tg of 33° C. was obtained. Further, theratio of side chains containing a carbon-carbon double bond was 0 andthe solid content was 50%.

<(Meth)Acrylic-Based Copolymer A15 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 18 pts. ofmethyl methacrylate, 78 pts. of n-butyl methacrylate, 4 pts. of2-hydroxyethyl methacrylate, and 100 pts. of toluene, and the mixturewas heated to 100° C. while being stirred in a nitrogen atmosphere.Then, 0.15 pts. of azobisisobutyronitrile was added and the mixtureunderwent a polymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone and 0.03 pts. of dibutyltindilaurate were added, and a solution obtained by dissolving 3.3 pts. of2-isocyanato ethyl methacrylate into 3.3 pts. of methyl ethyl ketone wasdropped in the mixture over two hours while stirring the mixture at 40°C. After it was confirmed that the isocyanate peak (2260 cm⁻¹) haddisappeared by IR measurement, a (meth)acrylic-based copolymer A15solution having a number average molecular weight of 37,000, a hydroxylvalue of 0 (mg KOH/g), and a Tg of 34° C. was obtained. Further, theratio of side chains containing a carbon-carbon double bond was 1/25 andthe solid content was 50%.

<(Meth)Acrylic-Based Copolymer A16 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 18 pts. ofmethyl methacrylate, 78 pts. of n-butyl methacrylate, 2 pts. of2-hydroxyethyl methacrylate, 2 pts. of glycidyl methacrylate, and 100pts. of toluene, and the mixture was heated to 100° C. while beingstirred in a nitrogen atmosphere. Then, 0.6 pts. ofazobisisobutyronitrile was added and the mixture underwent apolymerization reaction for 2 hours. Next, 0.05 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.05 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone, 0.8 pts. of dimethylbenzylamine, and 1 pt. of an acrylic acid were added and the mixture washeated and stirred for 15 hours at 100° C. After it was confirmed thatthe acid value was equal to or less than 2, a (meth)acrylic-basedcopolymer A16 solution having a number average molecular weight of14,000, a hydroxyl value of 17.0 (mg KOH/g), and a Tg of 31° C. wasobtained. Further, the ratio of side chains containing a carbon-carbondouble bond was 1/53 and the solid content was 50%.

<(Meth)Acrylic-Based copolymer A17 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 30 pts. ofn-butyl methacrylate, 66 pts. of 2-ethylhexyl methacrylate, 2 pts. of2-hydroxyethyl methacrylate, 2 pts. of glycidyl methacrylate, and 100pts. of toluene, and the mixture was heated to 100° C. while beingstirred in a nitrogen atmosphere. Then, 0.15 pts. ofazobisisobutyronitrile was added and the mixture underwent apolymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone, 0.8 pts. of dimethylbenzylamine, and 1 pt. of an acrylic acid were added and the mixture washeated and stirred for 15 hours at 100° C. After it was confirmed thatthe acid value was equal to or less than 2, a (meth)acrylic-basedcopolymer A17 solution having a number average molecular weight of35,000, a hydroxyl value of 18.3 (mg KOH/g), and a Tg of 0° C. wasobtained. Further, the ratio of side chains containing a carbon-carbondouble bond was 1/56 and the solid content was 50%.

<(Meth)Acrylic-Based Copolymer A18 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 63 pts. ofmethyl methacrylate, 33 pts. of n-butyl methacrylate, 2 pts. of2-hydroxyethyl methacrylate, 2 pts. of glycidyl methacrylate, and 100pts. of toluene, and the mixture was heated to 100° C. while beingstirred in a nitrogen atmosphere. Then, 0.15 pts. ofazobisisobutyronitrile was added and the mixture underwent apolymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone, 0.8 pts. of dimethylbenzylamine, and 1 pt. of an acrylic acid were added and the mixture washeated and stirred for 15 hours at 100° C. After it was confirmed thatthe acid value was equal to or less than 2, a (meth)acrylic-basedcopolymer A18 solution having a number average molecular weight of42,000, a hydroxyl value of 17.0 (mg KOH/g), and a Tg of 70° C. wasobtained. Further, the ratio of side chains containing a carbon-carbondouble bond was 1/63 and the solid content was 50%.

<(Meth)Acrylic-Based Copolymer A19 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 20 pts. ofmethyl methacrylate, 55 pts. of n-butyl methacrylate, 15 pts. of2-hydroxyethyl methacrylate, 10 pts. of glycidyl methacrylate, and 100pts. of toluene, and the mixture was heated to 100° C. while beingstirred in a nitrogen atmosphere. Then, 0.15 pts. ofazobisisobutyronitrile was added and the mixture underwent apolymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone, 0.8 pts. of dimethylbenzylamine, and 5 pts. of an acrylic acid were added and the mixturewas heated and stirred for 15 hours at 100° C. After it was confirmedthat the acid value was equal to or less than 2, a (meth)acrylic-basedcopolymer A19 solution having a number average molecular weight of37,000, a hydroxyl value of 108 (mg KOH/g), and a Tg of 40° C. wasobtained. Further, the ratio of side chains containing a carbon-carbondouble bond was 1/11 and the solid content was 51%.

<(Meth)Acrylic-Based Copolymer A20 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 20 pts. ofmethyl methacrylate, 53 pts. of n-butyl methacrylate, 27 pts. of2-hydroxyethyl methacrylate, and 100 pts. of toluene, and the mixturewas heated to 100° C. while being stirred in a nitrogen atmosphere.Then, 0.15 pts. of azobisisobutyronitrile was added and the mixtureunderwent a polymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone and 0.03 pts. dibutyltindilaurate were added, and a solution obtained by dissolving 21 pts. of2-isocyanato ethyl methacrylate into 21 pts. of methyl ethyl ketone wasdropped in the mixture over two hours while stirring the mixture at 40°C. After it was confirmed that the isocyanate peak (2260 cm⁻¹) haddisappeared by IR measurement, a (meth)acrylic-based copolymer A20solution having a number average molecular weight of 40,000, a hydroxylvalue of 8.0 (mg KOH/g), and a Tg of 43° C. was obtained. Further, theratio of side chains containing a carbon-carbon double bond was 1/4 andthe solid content was 50%.

<(Meth)Acrylic-Based Copolymer A21 Solution>

A four-necked flask equipped with a cooling tube, a stirrer, athermometer, and a nitrogen introducing tube was charged with 19.8 pts.of methyl methacrylate, 78 pts. of n-butyl methacrylate, 2 pts. of2-hydroxyethyl methacrylate, 0.2 pts. of glycidyl methacrylate, and 100pts. of toluene, and the mixture was heated to 100° C. while beingstirred in a nitrogen atmosphere. Then, 0.15 pts. ofazobisisobutyronitrile was added and the mixture underwent apolymerization reaction for 2 hours. Next, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours. Then, 0.07 pts. ofazobisisobutyronitrile was further added and the mixture underwent apolymerization reaction for 2 hours.

After that, 0.03 pts. of hydro-chinone, 0.8 pts. of dimethylbenzylamine, and 0.1 pts. of an acrylic acid were added and the mixturewas heated and stirred for 15 hours at 100° C. After it was confirmedthat the acid value was equal to or less than 2, a (meth)acrylic-basedcopolymer A21 solution having a number average molecular weight of37,000, a hydroxyl value of 8.8 (mg KOH/g), and a Tg of 34° C. wasobtained. Further, the ratio of side chains containing a carbon-carbondouble bond was 1/556 and the solid content was 50%.

<Polyisocyanate Compound Solution B>

A polyisocyanate compound solution (B) was obtained by diluting anisocyanurate of hexamethylene diisocyanate, which was blocked by3,5-dimethylpyrazole, to a concentration of 75% with ethyl acetate.

<Adjustment of Easy-Adhesive Solution>

Easy-adhesive solutions 1 to 26 were obtained by mixing a(meth)acrylic-based copolymer (A) solution and a polyisocyanate compound(B) solution in compositions shown in Table 2, and then mixing 0.01 wt.pts. of dioctyltin laurate based on 100 wt. pts. of the solid content ofthe (meth)acrylic-based copolymer (A) as a catalyst in each composition.

<Manufacturing of Solar Cell Rear Surface Protection Sheet>

Both surfaces of a polyester film (Tetoron (registered trademark) S,available from. Teijin DuPont Films Japan Limited, thickness: 188 μm)were corona-treated, and a polyester adhesive “Dinareo VA-3020/HD-701”(available from TOYOCHEM Co., Ltd., mixing ratio: 100/7, the sameapplies hereafter) was applied to one surface of the polyester film byusing a gravure coater. By drying the solvent, an adhesive layer havinga coating amount of 10 g/m² was provided. The vapor-deposition surfaceof the below-shown vapor-deposition PET (Teck Barrier LX available fromMitsubishi Plastics Inc., thickness: 12 μm) was placed over the adhesivelayer. After that, a polyester film/vapor-deposition PET laminate wasmanufactured by carrying out an aging process at 50° C. for four daysand thereby curing the adhesive layer.

Further, a polyester adhesive “Dinareo VA-3020/HD-701” (available fromTOYOCHEM Co., Ltd., mixing ratio: 100/7, the same applies hereafter) wasapplied to the vapor-deposition PET side surface of the polyesterfilm/vapor-deposition PET laminate by using a gravure coater. By dryingthe solvent, an adhesive layer having a coating amount of 10 g/m² wasprovided. A polyvinyl fluoride film (Tedlar available from Du PontKabushiki Kaisha, thickness: 50 μm) was placed over the adhesive layer.After that, a polyester film/vapor-deposition PET/polyvinyl fluoridefilm laminate was manufactured by carrying out an aging process at 50°C. for four days and thereby curing the adhesive layer.

Further, the easy-adhesive solution 1 was applied to the polyester filmsurface of the polyester film/vapor-deposition PET/polyvinyl fluoridefilm laminate by using a gravure coater. Then, a solar cell rear surfaceprotection sheet 1 was manufactured by drying the solvent and therebyproviding an easy-adhesive layer having a coating amount of 1 g/m².

Similarly to the solar cell rear surface protection sheet 1, solar cellrear surface protection sheets 2 to 26 were manufactured by using theeasy-adhesive 2 to 26.

A solar cell rear surface protection sheet 27 having a layer structurecomposed of a polyester film/vapor-deposition PET/polyvinyl fluoridefilm was manufactured without providing the easy-adhesive layer by usinga similar manufacturing method to that of the solar cell rear surfaceprotection sheet 1.

<Manufacture of Sample for Evaluating Adhesive Strength>

A white-plate glass, a vinyl acetate-ethylene copolymer film (standardcure type available from SANVIC inc., hereinafter “EVA film”), and thesolar cell rear surface protection sheet 1 were successively placed ontop of each other in such a manner that the easy-adhesive layer of thesolar cell rear surface protection sheet 1 comes into contact with theEVA film. After that this laminate was put into a vacuum laminator andthe air in the vacuum laminator was discharged so that the pressure wasreduced to 1 Torr. After the laminate was heated for 30 minutes at 150°C. under a pressure of 0.1 MPa, the laminate was further heated for 30minutes at 150° C. As a result, a 10 cm square sample 1 for evaluatingan adhesive strength was manufactured.

Similarly to the sample 1 for evaluating an adhesive strength, samples 2to 26 for evaluating an adhesive strength were manufactured by using thesolar cell rear surface protection sheets 2 to 26.

Similarly to the sample 1 for evaluating an adhesive strength, a sample27 for evaluating an adhesive strength was manufactured by successivelyplacing a white-plate glass, an EVA film, and the solar cell rearsurface protection sheet 27 on top of each other in such a manner thatthe polyester film side surface of the solar cell rear surfaceprotection sheet 27 comes into contact with the EVA film.

Example 1

By using the sample 1 for evaluating an adhesive strength, the adhesiveproperty of the easy-adhesive layer with the EVA film and the adhesiveproperty after moisture/heat resistance tests (500 hours, 1000 hours,and 2000 hours) were evaluated by using the below-described method.

<Measurement of Adhesive Property>

The solar cell rear surface protection sheet 1 surface of the sample 1for evaluating an adhesive strength was cut into a strip(s) having awidth of 15 mm, and the adhesive strength between the easy-adhesivelayer formed in the solar cell rear surface protection sheet 1 and theEVA film was measured. For the measurement, a 180-degree peel test wascarried out at a load speed of 100 mm/min by using a tensile testingdevice. Evaluations were made for obtained measurement values in thefollowing manner:

⊚: no less than 50N/15 mm;

◯: no less than 30N/15 mm and less than 50N/15 mm;

Δ: no less than 10N/15 mm and less than 30N/15 mm; and

x: less than 10N/15 mm.

<Adhesive Property After Moisture/Heat Resistance Test>

Similarly to the adhesive property measurement, the adhesive property ofthe sample 1 for evaluating an adhesive strength was evaluated againafter moisture/heat resistance tests, i.e., after the sample 1 was leftuntouched for 500 hours, 1000 hours, and 2000 hours in an environment ofa temperature of 85*° C. and a relative humidity of 85% RH.

Examples 2 to 16, Comparative Examples 1 to 10

Similarly to Example 1, the adhesive property of the easy-adhesive layerwith the EVA film and the adhesive property after moisture/heatresistance tests were evaluated by using samples 2 to 26 for evaluatingan adhesive strength.

Comparative Example 11

By using the sample 27 for evaluating an adhesive strength, which wasmanufactured without using the easy-adhesive solution, the adhesiveproperty between the polyester film surface and the EVA film and theadhesive property after moisture/heat resistance tests were evaluated.Table 2 shows the results of the above-described evaluations.

TABLE 1 Ratio of Number side chains average conaining molecular OH valuecarbon-carbon Composition Tg weight mgKOH/g double bond A1 MMA/n-BMA/HEMA/GMA (AA denaturation) = 18/78/2/2 30 39000 17.2 1/53  A2 MMA/n-BMA/HEMA/GMA (AA denaturation) = 18/78/2/2 30 26000 16.5 1/53  A3 MMA/n-BMA/HEMA/GMA (AA denaturation) = 18/78/2/2 30 75000 18.0 1/53  A4 MMA/n-BMA/HEMA/HEMA (MOI denaturation) = 18/78/2/2 30 38000 8.6 1/49 A5  MMA/n-BMA/HEMA/HEMA (MOI denaturation) = 18/78/0.5/3.5 27 33000 2.11/28  A6  MMA/n-BMA/HEMA/MIAH (HEMA denaturation) = 18/78/2/2 30 360008.5 1/37  A7  n-BMA/HEMA/GMA (AA denaturation) = 96/2/2 20 38000 17.01/50  A8  MMA/n-BMA/HEMA/GMA (AA denaturation) = 41/55/2/2 51 41000 8.51/59  A9  MMA/n-BMA/HEMA/GMA (AA denaturation) = 20/63/15/2 40 4500073.0 1/55  A10 MMA/n-BMA/HEMA/GMA (AA denaturation) = 45/38/2/15 5737000 70.2 1/8  A11 MMA/n-BMA/HEMA/GMA (AA denaturation) = 19.5/78/2/0.534 35000 10.5 1/222 A12 MMA/n-BMA/HEMA/HEMA (MOI denaturation) =18/78/2/2 30 242000 8.0 1/49  A13 MMA/n-BMA/HEMA/HEMA (MOI denaturation)= 20/55/23/2 42 44000 98.7 1/51  A14 MMA/n-BMA/HEMA = 18/80/2 33 380008.6 0 A15 MMA/n-BMA/HEMA (MOI denaturation) = 18/78/4 34 37000 0 1/25 A16 MMA/n-BMA/HEMA/GMA (AA denaturation) = 18/78/2/2 31 14000 17.0 1/53 A17 n-BMA/2-EHMA/HEMA/GMA (AA denaturation) = 30/66/2/2 0 35000 18.31/56  A18 MMA/n-BMA/HEMA/GMA (AA denaturation) = 63/33/2/2 70 42000 171/63  A19 MMA/n-BMA/HEMA/GMA (AA denaturation) = 20/55/15/10 40 37000108 1/11  A20 MMA/n-BMA/HEMA/HEMA (MOI denaturation) = 20/53/2/25 4340000 8.0 1/4  A21 MMA/n-BMA/HEMA/GMA (AA denaturation) = 19.8/78/2/0.234 37000 8.8 1/556

TABLE 2 Solar cell rear surface protection sheet Easy-adhesive solutionAcrylic-based copolymer (A) Polyisocyanate compound (B) Contact propertySide chain Mixing Moisture/heat Moisture/heat OH having C=C amountNCO/OH resistance test resistance test Tg Mn value (*1) (*2) ratioInitial After 1000 hr. After 2000 hr. Examples 1 1 A1 30 39000 17.2 1/5318 2 ⊚ ⊚ ⊚ 2 2 A2 30 26000 16.5 1/53 18 2 ⊚ ⊚ ◯ 3 3 A3 30 75000 18 1/5319 2 ⊚ ⊚ ⊚ 4 4 A4 30 38000 8.6 1/49 9 2 ⊚ ⊚ ⊚ 5 5 A5 27 33000 2.1 1/28 22 ⊚ ⊚ ⊚ 6 6 A6 30 36000 8.5 1/37 9 2 ⊚ ⊚ ◯ 7 7 A7 20 38000 17 1/50 10 2⊚ ⊚ ◯ 8 8 A8 51 41000 8.5 1/59 9 2 ◯ ◯ ◯ 9 9 A9 40 45000 73 1/55 78 2 ◯◯ ◯ 10 10 A10 57 37000 70.2 1/8  75 2 ◯ ◯ ◯ 11 11 A11 34 35000 10.5 1/222 11 2 ◯ ◯ ◯ 12 12 A1 30 39000 17.2 1/53 5 0.5 ⊚ ⊚ ◯ 13 13 A1 9 1 ⊚⊚ ⊚ 14 14 A1 36 4 ⊚ ⊚ ⊚ 15 15 A12 30 242000 8 1/49 9 2 ◯ ◯ ◯ 16 16 A1342 44000 98.7 1/51 52 1 ◯ ◯ Δ Comparative 1 17 A14 33 38000 8.6 0 9 2 ΔΔ X examples 2 18 A15 34 37000 0 1/25 0 0 ◯ X X 3 19 A16 31 14000 171/53 18 2 ◯ Δ X 4 20 A17 0 35000 18.3 1/56 20 2 Δ Δ X 5 21 A18 70 4200017 1/63 18 2 Δ Δ X 6 22 A19 40 37000 108 1/11 116 2 Δ X X 7 23 A20 4340000 8 1/4  8.5 2 X X X 8 24 A21 34 37000 8.8  1/556 9.5 2 ◯ Δ X 9 25A1 30 39000 17.2 1/53 0 0 ◯ X X 10 26 A1 90 10 Δ X X 11 without adhesive◯ X X (*1) Side chain having C=C . . . ratio of side chains having C=Cto all side chains (*2) Mixing ratio . . . mixing amount (wt. pts.) ofeach polyisocyanate compound (B) based on 100 wt. pts. of solid contentof each copolymer (A) solution *3 0.01 wt. pts. of dioctyltin laurate ismixed based on 100 wt. pts. of solid content of each copolymer (A) as acatalyst in each composition.

As shown in Table 2, Examples 1 to 16, which are solar cell rear surfaceprotection sheets using an easy-adhesive according to the presentinvention, have a satisfactory adhesive property and an adhesiveproperty after moisture/heat resistance tests.

In contrast to this, Example 1 is poor in the adhesive property becausethe (meth)acrylic-based copolymer does not contain the carbon-carbondouble bond in the side chain.

As for Example 2, since the OH value is less than two, the cross-linkingis insufficient and thus the adhesive property after the moisture/heatresistance test is poor.

As for Example 3, the number average molecular weight of theacrylic-based copolymer is so small that the adhesive property after themoisture/heat resistance test is poor.

As for Example 4, the Tg of the (meth)acrylic-based copolymer (A) is solow that the cohesive force is small. As a result, the adhesive propertyis poor. As for Example 5, the Tg of the (meth)acrylic-based copolymer(A) is so high that the easy-adhesive layer (D′) becomes harder. As aresult, the adhesive property is poor.

As for Example 6, the OH value of the acrylic-based copolymer is greaterthan 100. As for Example 7, the ratio of side chains containing acarbon-carbon double bond of the acrylic-based copolymer is so high thatthe cross-linking is excessive. As a result, the adhesive property ispoor.

As for Example 8, since the ratio of side chains containing acarbon-carbon double bond of the acrylic-based copolymer is excessivelyhigh, a satisfactory adhesive property cannot be obtained.

As for Example 9, since the NCO/OH ratio of the acrylic-based copolymerand the curing agent is zero, the adhesive property is poor after themoisture/heat resistance test. As for Example 10, the NCO/OH ratio is 10and thus the cross-linking is excessive. As a result, the adhesiveproperty and the adhesive property after the moisture/heat resistancetest are poor.

Example 17 Manufacture of Solar Cell Module

-   White-plate glass . . . solar cell front surface protection material    (I)-   EVA film . . . light-receiving-side sealing agent (II)-   Polycrystalline silicon solar cell element . . . solar battery cell    (III)-   EVA film . . . non-light-receiving-side sealing agent (IV)

After the above-listed elements (I) to (IV) and a solar cell rearsurface protection sheet 1 were successively placed on top of each otherin such a manner that the easy-adhesive layer of the solar cell rearsurface protection sheet 1 comes into contact with thenon-light-receiving-side sealing agent (IV), the laminate was put into avacuum laminator and the air in the vacuum laminator was discharged sothat the pressure was reduced to 1 Torr. After the laminate was heatedfor 30 minutes at 150° C. under the atmospheric pressure, which was usedas the press pressure, the laminate was further heated 30 minutes at150° C. As a result, a 10 cm square solar cell module 1 for evaluatingoptical/electrical conversion efficiency was manufactured.

<Measurement of Optical/Electrical Conversion Efficiency>

The solar cell output of the obtained solar cell module 1 was measuredin order to measure optical/electrical conversion efficiency by using asolar simulator (SS-100XIL available from Eko Instruments Co., Ltd.) inaccordance with JIS C8912.

Further, the optical/electrical conversion efficiency was also measuredin a similar manner after moisture/heat resistance tests, i.e., afterthe solar cell module 1 was left untouched for 500 hours, 1000 hours,1500 hours, and 2000 hours in an environment of a temperature of 85° C.and a relative humidity of 85% RH. Evaluations were made by calculatingrates at which the optical/electrical conversion efficiency deterioratesfrom the initial optical/electrical conversion efficiency to theoptical/electrical conversion efficiency after the moisture/heatresistance test in the following manner:

◯: output deterioration is less than 10%;

Δ: output deterioration is no less than 10% and less than 15%; and

×: output deterioration is no less than 20%.

Examples 18 to 23, Comparative Examples 12 to 16

Similarly to Example 17, solar cell modules 2 to 12 were manufactured byusing the solar cell rear surface protection sheets 2 to 7, 17, 19, 20,23 and 24, and their optical/electrical conversion efficiencies(initial, after moisture/heat resistance test) were measured.

Comparative Example 17

Comparative example 17 was manufactured in a similar manner to that ofExample 17 except that the solar cell rear surface protection sheet 27was used in place of the solar cell rear surface protection sheet 1 andwas laminated in such a manner that the polyester film surface of thesolar cell rear surface protection sheet 27 comes into contact with thenon-light-receiving-side sealing agent (IV). Further, itsoptical/electrical conversion efficiencies (initial, after moisture/heatresistance test) were measured. Table 3 shows the results of theabove-described evaluations.

TABLE 3 Solar cell module Deterioration in output after moisture/heatresistance test Solar cell Moisture/heat resistance test (85° C. 85%)Solar cell rear surface after after after after module protection sheet500 hr. 1000 hr. 1500 hr. 2000 hr. Examples 17 1 1 ◯ ◯ ◯ ◯ 18 2 2 ◯ ◯ ◯◯ 19 3 3 ◯ ◯ ◯ ◯ 20 4 4 ◯ ◯ ◯ ◯ 21 5 5 ◯ ◯ ◯ ◯ 22 6 8 ◯ ◯ ◯ ◯ 23 7 7 ◯ ◯◯ ◯ Comparative 12 8 17 Δ Δ X X examples 13 9 19 ◯ Δ X X 14 10 20 Δ X XX 15 11 23 X X X X 16 12 24 ◯ Δ X X 17 13 27 Δ X X X

As shown in Table 3, no deterioration in the output performance wasobserved for of Examples 17 to 23. In contrast, in Comparative examples12 to 17, since the adhesive property between the EVA film and the solarcell rear surface protection sheet is insufficient, the solar cellelement has deteriorated due to the moisture infiltration. As a result,the optical/electrical conversion efficiency deteriorates.

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2010-154392, filed on Jul. 7, 2010, thedisclosure of which is incorporated herein in its entirety by reference.

REFERENCE SIGNS LIST

-   I SOLAR CELL FRONT SURFACE PROTECTION MATERIAL LOCATED ON    LIGHT-RECEIVING SIDE OF SOLAR BATTERY CELL-   II SEALING AGENT LOCATED ON LIGHT-RECEIVING SIDE OF SOLAR BATTERY    CELL-   III SOLAR BATTERY CELL-   IV SEALING AGENT LOCATED ON NON-LIGHT-RECEIVING SIDE OF SOLAR    BATTERY CELL-   V SOLAR CELL REAR SURFACE PROTECTION SHEET

The invention claimed is:
 1. An adhesive composition for a solar cellrear surface protection sheet, comprising: a (meth)acrylic-basedcopolymer (A) having a glass transition temperature of 10-60° C., anumber average molecular weight of 25,000-250,000, and a hydroxyl valueof 2-100 (mgKOH/g), and containing one carbon-carbon double bond forevery 5-500 side chains; and a polyisocyanate compound (B), thepolyisocyanate compound (B) being contained in such an amount that anumber of an isocyanate group is 0.1-5 for every one hydroxyl groupcontained in the (meth)acrylic-based copolymer (A), wherein the curedcomposition comprises an adhesive peel strength after 2000 hours at 85°C. and 85 percent humidity of no less than 30 N per 15 mm.
 2. Theadhesive composition for a solar cell rear surface protection sheetaccording to claim 1, wherein the (meth)acrylic-based copolymer (A) is acopolymer selected from a group consisting of following(meth)acrylic-based copolymers (A1) to (A4): a (meth)acrylic-basedcopolymer (A1): a (meth)acrylic-based copolymer that is formed byreacting a (meth)acrylic-based monomer containing a carboxyl group (a3)with a glycidyl group of a side chain contained in a copolymercontaining a (meth)acrylic-based monomer containing a glycidyl group(a1), a (meth)acrylic-based monomer containing a hydroxyl group (a2),and a (meth)acrylic-based monomer containing no glycidyl group, nohydroxyl group, and no carboxyl group (a4) as constitutional units; a(meth)acrylic-based copolymer (A2): a (meth)acrylic-based copolymer thatis formed by reacting a (meth)acrylic-based monomer containing aglycidyl group (a1) with a carboxyl group contained in a copolymercontaining a (meth)acrylic-based monomer containing a carboxyl group(a3), a (meth)acrylic-based monomer containing a hydroxyl group (a2),and a (meth)acrylic-based monomer containing no glycidyl group, nohydroxyl group, and no carboxyl group (a4) as constitutional units; a(meth)acrylic-based copolymer (A3): a (meth)acrylic-based copolymer thatis formed by reacting a (meth)acrylic-based monomer containing anisocyanate group (a5) with a part of a hydroxyl group contained in acopolymer containing a (meth)acrylic-based monomer containing a hydroxylgroup (a2) and a (meth)acrylic-based monomer containing no hydroxylgroup (a6) as constitutional units; and a (meth)acrylic-based copolymer(A4): a (meth)acrylic-based copolymer that is formed by reacting a(meth)acrylic-based monomer containing a hydroxyl group (a2) with anacid anhydride group contained in a copolymer containing a maleicanhydride, a (meth)acrylic-based monomer containing a hydroxyl group(a2), and a (meth)acrylic-based monomer containing no hydroxyl group(a6) as constitutional units.
 3. The adhesive composition for a solarcell rear surface protection sheet according to claim 1, wherein thepolyisocyanate compound (B) is a blocked polyisocyanate compound (B1).4. A solar cell rear surface protection sheet comprising: an uncuredadhesive comprising: a (meth)acrylic-based copolymer (A) having a glasstransition temperature of 10-60° C., a number average molecular weightof 25,000-250,000, and a hydroxyl value of 2-100 (mgKOH/g), andcontaining one carbon-carbon double bond for every 5-500 side chains;and a polyisocyanate compound (B), the polyisocyanate compound (B) beingcontained in such an amount that a number of an isocyanate group is0.1-5 for every one hydroxyl group contained in the (meth)acrylic-basedcopolymer (A), wherein the cured adhesive composition comprises anadhesive peel strength after 2000 hours at 85° C. and 85 percenthumidity of no less than 30 N per 15 mm; and a plastic film.
 5. A solarcell module comprising: a solar battery cell; a solar cell front surfaceprotection material that protects the solar battery cell through asealing agent disposed on a light-receiving side, the solar cell frontsurface protection material being disposed on the light-receiving side;and a solar cell rear surface protection sheet that protects the solarbattery cell through a sealing agent disposed on a non-light-receivingside, the solar cell rear surface protection sheet being disposed on thenon-light-receiving side, wherein the solar cell rear surface protectionsheet is obtained by disposing a solar cell rear surface protectionsheet comprising a plastic film and an uncured adhesive layercomprising: a (meth)acrylic-based copolymer (A) having a glasstransition temperature of 10-60° C., a number average molecular weightof 25,000-250,000, and a hydroxyl value of 2-100 (mgKOH/g), andcontaining one carbon-carbon double bond for every 5-500 side chains;and a polyisocyanate compound (B), the polyisocyanate compound (B) beingcontained in such an amount that a number of an isocyanate group is0.1-5 for every one hydroxyl group contained in the (meth)acrylic-basedcopolymer (A), in such a manner that the adhesive layer comes intocontact with the sealing agent disposed on the non-light-receiving side,and curing the adhesive layer wherein the cured composition comprises anadhesive peel strength after 2000 hours at 85° C. and 85 percenthumidity of no less than 30 N per 15 mm.
 6. The solar cell moduleaccording to claim 5, wherein the sealing agent disposed on thenon-light-receiving side contains an organic peroxide.
 7. The solar cellmodule according to claim 5, wherein the sealing agent disposed on thenon-light-receiving side contains an ethylene-vinyl acetate copolymer(EVA) as a main component.